1. Field of the Invention
The invention relates to an abrasives composition. This invention particularly relates to an abrasives composition capable of producing a substrate for a magnetic recording medium, which substrate has a very small value of surface roughness. This invention also relates to a process for producing a substrate, wherein the abrasives composition is used. This invention further relates to a glass-like carbon substrate for a magnetic recording medium, wherein the substrate has a very small value of surface roughness, and a magnetic recording medium using the glass-like carbon substrate.
2. Description of the Related Art
Recently, most computers are provided with recording devices, such as magnetic disk drives. With the increase in the amount of recorded information occurring in recent years, it is required that recording devices be capable of recording information at high densities. The magnetic disk drive has a magnetic head and a magnetic disk. As a means for achieving the recording of information at high densities, it is necessary to reduce the glide height of the magnetic head. For such purposes, it is necessary that the surface roughness (Ra) of the magnetic disk be as small as possible.
Recently, glass-like carbon substrates have attracted particular attention as substrates for magnetic disks. The glass-like carbon substrates have a hardness higher than aluminum substrates, which are currently popular as the substrates for magnetic disks. Therefore, the thickness of the substrates can be reduced when made of glass-like carbon. Also, glass-like carbon has a small specific gravity and is light in weight, and therefore glass-like carbon substrates are light in weight. Accordingly, substrates are very suitable for use in computers, which should be kept small in size and light in weight.
However, when glass-like carbon substrates are used as substrates for magnetic disks, it is difficult for the surface roughness of the glass-like carbon substrates to be reduced, due to the high hardness of the glass-like carbon. Specifically, if .gamma.-alumina particles, typically used, are used in polishing step carried out during the production of the substrates for magnetic disks, the surface roughness of the substrates cannot be reduced (e.g., to less than 4 .ANG.). This is because the .alpha.-alumina particles are hard and the mean particle size of their primary particles is at least 0.1 .mu.m. If very soft abrasive is used in lieu of .alpha.-alumina particles, sufficient abrasive effects cannot be obtained, and the surface roughness cannot be reduced.
Japanese Patent Application Laid-Open 7-240025 proposes a method for reducing the surface roughness of a magnetic disk substrate. The method comprises the step of corroding a substrate material with a chemical corrosive agent, which reacts with the substrate material and softens a portion of the substrate material, and the step of abrading and removing this portion of the substrate material using colloidal particles. Also proposed is a magnetic disk substrate having a surface roughness of less than 4 .ANG.. However, it is difficult to reduce the surface roughness of a glass-like carbon substrate to less than 4 .ANG. with the method proposed in this patent application.